Magnetic Coupling for Downhole Applications

ABSTRACT

The present disclosure relates to a magnetic coupling of a downhole tool that includes a first annular array of magnetic sections, a second annular array of magnetic sections coupled to the first annular array by a magnetic field that transfers rotational motion from the first annular array to the second annular array, and a barrier disposed between the first annular array and the second annular array, the barrier including an erosion-resistant layer. The present disclosure also relates to a method of bootstrapping a magnetic coupling of a downhole tool. The method includes supplying electrical current from a battery to an electromagnetic coil in the magnetic coupling, transferring rotational motion from the magnetic coupling to an alternating current (AC) source and supplying electrical current from the AC source to the electromagnetic coil.

TECHNICAL FIELD

The present disclosure relates generally to drilling tools and, moreparticularly, to magnetic coupling for downhole applications.

BACKGROUND

Natural resources, such as oil and gas, often reside in various formswithin a subterranean geological formation that may be located onshoreor offshore. These natural resources can be recovered by drilling awellbore that penetrates the formation.

A variety of fluids are used in both drilling and completing thewellbore. For example, during the drilling of the wellbore, a portion ofthe drill string may be immersed in a drilling fluid used to cool thedrill bit, lubricate the rotating drill string to prevent it fromsticking to the walls of the wellbore, prevent blowouts by serving as ahydrostatic head to the entrance into the wellbore of formation fluids,and remove drill cuttings from the wellbore, among other uses. Otherportions of the drill string may be immersed in oil, in air, or in othersuitable media.

The drill string used in such drilling operations may include a varietyof components. Some components may capture energy from the flow ofdrilling fluid through the drill string. For example, the drill stringmay include a turbine that produces rotational motion. Other componentsmay make use of such rotational motion. For example the drill string mayinclude a pump driven by a swash plate, an actuator such as a ballscrew, or a generator that produces electrical current used to operateother equipment within the drill string. Such other equipment mayinclude sensors, telemetry components, measurement while drilling (MWD)tools, logging-while-drilling (LWD) tools, or other components.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is an elevation view of an exemplary drilling system;

FIG. 2 is a section view of a portion of an exemplary drill stringcontaining a magnetic coupling;

FIG. 3 is a perspective view of an exemplary axial magnetic coupling;

FIG. 4 is an elevation section view of an exemplary radial magneticcoupling;

FIG. 5 is a plan section view of an exemplary radial magnetic coupling;

FIG. 6 is a circuit diagram of an exemplary bootstrap circuit forenergizing electromagnetic coils in a magnetic coupling; and

FIG. 7 is a flow chart of an exemplary method for bootstrapping amagnetic coupling.

DETAILED DESCRIPTION

The present disclosure describes magnetic couplings used to transfermechanical power from a prime mover such as a turbine or other source ofrotational motion to a load that includes equipment that can make use ofthat motion, such as a generator. The turbine is driven by the flow ofdrilling fluid. A barrier is interposed between the drive side of themagnetic coupling and the follower side of the magnetic coupling toprevent the drilling fluid, which is electrically conductive, frominterfering with the operation of the load. The magnetic couplingincludes a pair of Halbach arrays, each of which includes a series ofmagnetic sections arranged to enhance the magnetic field in the spacebetween the two arrays while diminishing or eliminating the field on theopposite side of each array. As a result, the amount of torque that istransmitted by the magnetic coupling is greater than the torquetransmitted by a coupling using conventional magnetic arrays. Themagnetic coupling may use electromagnetic coils in the place of somepermanent magnets, allowing the strength of the magnetic coupling to betuned to meet varying operational requirements. Additionally, the powerused to energize the electromagnetic coils in the magnetic coupling maybe supplied by the load driven by the coupling, once the coupling beginsto turn.

Embodiments of the present disclosure and its advantages are bestunderstood by referring to FIGS. 1 through 7, where like numbers areused to indicate like and corresponding parts.

FIG. 1 is an elevation view of an exemplary drilling system. Drillingsystem 100 includes well surface or well site 106. Various types ofdrilling equipment such as a rotary table, drilling fluid pumps anddrilling fluid tanks (not expressly shown) are located at well surfaceor well site 106. For example, well site 106 may include drilling rig102 that has various characteristics and features associated with a“land drilling rig.” However, drilling systems incorporating teachingsof the present disclosure may be satisfactorily used with drillingequipment located on offshore platforms, drill ships, semi-submersiblesand drilling barges (not expressly shown). Well site 106 and drillingrig 102 are located above subterranean region 107.

Drilling system 100 also includes drill string 103 associated with drillbit 101 that may be used to form a wide variety of wellbores or boreholes such as wellbore 114. A portion of wellbore 114 that is closer towell surface 106 is referred to as uphole, and a portion of wellbore 114that is further from well surface 106 is referred to as downhole.Wellbore 114 may be defined in part by casing string 110 that extendsfrom well surface 106 to a selected downhole location. Portions ofwellbore 114 that do not include casing string 110 are described as openhole.

Various drilling fluids are used during the drilling of wellbores. Thedrilling fluid serves many purposes, including cooling the drill bit,lubricating the rotating drill string to prevent it from sticking to thewalls of the wellbore, preventing blowouts by serving as a hydrostatichead to the entrance into the wellbore of formation fluids, and removingdrill cuttings from the wellbore. Typically the drilling fluid iscirculated downward through drill string 103 and drill bit 101 and thenmoves upward through the wellbore towards the surface through annulus108. In open hole embodiments, annulus 108 is defined in part by outsidediameter 112 of drill string 103 and inside diameter 118 of wellbore114. In embodiments using casing string 110, annulus 108 is defined byoutside diameter 112 of drill string 103 and inside diameter 111 ofcasing string 110. Other circulation pathways are possible, however.Drilling fluid typically includes a base fluid, for example water orsalt water, mixed with other materials or additives. As a result,drilling fluid is often electrically conductive.

Drill string 103 may include a wide variety of components configured toform wellbore 114. For example, components 122 a, 122 b, and 122 c ofdrill string 103 may include, but are not limited to, drill bits (e.g.,drill bit 101), coring bits, drill collars, rotary steering tools,directional drilling tools, downhole drilling motors, turbines, magneticcouplings, generators, reamers, hole enlargers, stabilizers, sensors,logging-while-drilling tools, or telemetry subs. The number and types ofcomponents 122 included in drill string 103 depend on anticipateddownhole drilling conditions and the type of wellbore that will beformed by drill string 103 and rotary drill bit 101. Drill string 103may also include one or more electrically powered components, such assensors, logging-while-drilling (LWD) tools, controllers, telemetrysubs, communication components, well logging instruments, and downholetools associated with directional drilling of a wellbore.

Drill string 103 may also include components to provide the electricalcurrent required to operate components of the drill string, such ascomponent 122 c discussed above. For example, component 122 a mayinclude a turbine or other type of motor immersed in the drilling fluidwithin drill string 103. The turbine is configured to transform the flowof the drilling fluid through drill string 103 into rotational motion ofa drive shaft. Component 122 b may include a generator configured totransform the rotational motion of the drive shaft into electricalcurrent for use by other components, such as component 122 c. Althoughthis disclosure describes specific components 122 a, 122 b, and 122 c,any suitable components of a drill string may be used. Furthermore,although this disclosure discusses a particular arrangement ofcomponents 122 a, 122 b, and 122 c, components of drill string 103 maybe arranged in any suitable positions within drill string 103.

Because drilling fluid is often electrically conductive, immersion ofthe generator in the drilling fluid may interfere with the operation ofthe generator. Therefore, the generator is contained within aload-enclosure portion of drill string 103 that contains oil, air, orother suitable non-conductive media, and is separated from the drillingfluid by a barrier such as a static seal that does not rotate relativeto drill string 103 and is not penetrated by the drive shaft. A magneticcoupling, as shown in further detail in FIGS. 2-6, is used to transferrotational motion across the barrier, from the portion of the driveshaft connected to the turbine and immersed in the drilling fluid to theportion of the drive shaft connected to the generator and immersed inthe non-conductive media. The load enclosure allows drilling fluid toflow towards the downhole end of drill string 103. For example, the loadenclosure may be narrower than the inner diameter of drill string 103,allowing drilling fluid to flow downhole between the load enclosure andthe inner diameter of drill string 103.

Drill bit 101 typically includes one or more blades 126 located onexterior portions of rotary bit body 124 of drill bit 101. Blades 126are any suitable type of projections extending outwardly from rotary bitbody 124. Drill bit 101 rotates with respect to bit rotational axis 104in a direction defined by directional arrow 105. Blades 126 include oneor more cutting elements 128 located on exterior portions of each blade126. Blades 126 may also include one or more depth of cut controllers(not expressly shown) configured to control the depth of cut of cuttingelements 128. Blades 126 may further include one or more gage pads (notexpressly shown) located on blades 126. Drill bit 101 may have manydifferent designs, configurations, and/or dimensions according to theparticular application of drill bit 101.

Drilling system 100 may include additional or different features, andthe features of drilling system 100 may be arranged as shown in FIG. 1,or in another suitable configuration.

FIG. 2 is a section view of a portion 200 of an exemplary drill stringcontaining a magnetic coupling. Drill string 103 includes one or moresegments of drill pipe 116 whose inner diameter defines throat 208.Turbine 202 is located within throat 208. Turbine 202 may be a motor orany apparatus that produces rotational motion. For example, turbine 202as illustrated in FIG. 2 is an axial flow turbine including an impellerlocated in throat 208 of drill pipe 116 and configured to capture thekinetic energy of drilling fluid flow to produce rotational motion.Turbine 202 has several blades 204 distributed around the periphery andangled relative to the axis of drill pipe 116. Although turbine 202 isillustrated in FIG. 2 as an axial flow turbine, turbine 202 may includea transverse-flow turbine in which fluid flow through the turbine issubstantially perpendicular to the rotational axis of the impeller.Turbine 202 is coupled to drive shaft 210, which runs parallel to axis226 of throat 208. Drive shaft 210 is coupled to drive array 222 ofmagnetic coupling 220.

Magnetic coupling 220 includes drive array 222 and follower array 224,separated by barrier 230. Each of drive array 222 and follower array 224includes an annular array of magnetic sections, which may includepermanent magnets or electromagnetic coils, arranged in a Halbach array.In a Halbach array, sections of the array that produce a magnetic fluxoriented normal to the surface of the array alternate with sections thatproduce a magnetic flux oriented transverse to the surface of the array.As a result of this arrangement of magnetic fluxes, the magnetic fieldbetween drive array 222 and follower array 224 is stronger than usingonly array sections that produce fluxes normal to the surface. As aresult, magnetic coupling 220 is capable of transferring higher levelsof torque.

As illustrated in FIG. 2, magnetic coupling 220 is an axial coupling, inwhich the two arrays are of substantially similar diameter.Specifically, drive array 222 includes an annular array of diameter 221,with the magnetic sections arranged in a circle about axis of rotation226. Follower array 224 also includes an annular array of diameter 221,with the magnetic sections arranged in a circle about axis of rotation226. Drive array 222 and follower array 224 are coupled by magneticfield 228, which penetrates barrier 230. The arrangement of magneticsections and magnetic fields in drive array 222 and follower array 224is described in more detail in connection with FIG. 3 below.

Follower array 224 is coupled to follower shaft 240. Follower shaft 240is coupled to load 250, for example an electrical generator.

Load enclosure 252 encloses follower array 224, follower shaft 240, andload 250. Barrier 230, located between drive array 222 and followerarray 224, separates drilling fluid in throat 208 from oil, air, orother suitable non-conductive media within load enclosure 252. Magneticfield 228 between drive array 222 and follower array 224 penetratesbarrier 230 to couple the two arrays. Barrier 230 may be coupled to theinterior surface of drill pipe 116 by stays 232, so that barrier 230does not rotate with drive array 222 or follower array 224. Barrier 230may be composed of a variety of materials in one or more layers. Forexample, Barrier 230 may include one or more layers of ceramic, polymersor thermoplastics such as polyether ether ketone (PEEK), composites suchas fiberglass, or other suitable materials. In some embodiments, thesurface of barrier 230 in contact with drilling fluid includes a layerof titanium, which is resistant to erosion from the flow of drillingfluid. The layer of titanium included in barrier 230 may be very thin tolimit the strength of eddy currents that are induced in the titaniumlayer by the changing magnetic flux produced by the motion of drivearray 222 and follower array 224 relative to barrier 230.

Although turbine 202 is illustrated in FIG. 2 as being located upholefrom magnetic coupling 220 and load 250, these components of portion 200may be located in any suitable arrangement. For example, turbine 202 maybe located downhole from magnetic coupling 220 and load 250. As anotherexample, turbine 202 may be located outside the outer diameter of drivearray 222.

In operation, drilling fluid flows through throat 208 in the directionindicated by arrow 205. As it flows, the drilling fluid contacts blades204 of turbine 202. Because blades 204 are angled with respect to theflow of drilling fluid, the drilling fluid pushes against blades 204 andcauses turbine 202 to spin, producing rotational motion. This rotationalmotion is transferred to drive array 222 by drive shaft 210. Becausedrive array 222 is magnetically coupled to follower array 224 ofmagnetic coupling 220 by magnetic field 228, the rotational motion ofdrive array 222 is transferred to follower array 224 across barrier 230.Follower array 224 is coupled to load 250 through follower shaft 240. Asa result, the rotational motion produced by turbine 202 is transmittedto load 250.

Load 250, located within load enclosure 252, utilizes the rotationalmotion of follower shaft 240. In some embodiments, load 250 may be agenerator that transforms the rotational motion of follower shaft 240into electrical current. For example, load 250 may be a permanent magnetalternating current generator, a transverse flux generator, a radialflux generator, an axial flux generator, a direct current generator, analternator, or any other suitable type of generator. In someembodiments, load 250 may be a pump that transforms the rotationalmotion of follower shaft 240 into reciprocal motion of one or morepistons through the use of a swash plate. In some embodiments, load 250may include an actuator that transforms the rotational motion offollower shaft 240 into linear motion, for example through the use of aball screw. Although the present disclosure discusses particularexamples of load 250, any suitable load that makes use of rotationalmotion of follower shaft 240 may be used.

After drilling fluid passes turbine 202, it continues through throat 208and around barrier 230 into fluid passage 234. For example, fluidpassage 234 may be an annulus between barrier 230 and drill pipe 116.From fluid passage 234, drilling fluid continues downhole toward drillbit 101, as discussed in connection with FIG. 1.

In embodiments that include electromagnetic coils, the current suppliedto the electromagnetic coils may be tuned to vary the amount of magneticflux produced by each coil. In some embodiments, the current supplied tothe electromagnetic coils is lower when follower array 224 and followershaft 240 are turning than when follower array 224 and follower shaft240 are not turning. For example, when drive shaft 210 first begins torotate, for example when turbine 202 is started, a large torque istypically required to cause follower shaft 240 to begin rotating todrive load 250. Under such circumstances, the power supplied to theelectromagnetic coils may be approximately 75% to 80% of the maximumpower P_(mx) that can be provided to the electromagnetic coils, allowingmagnetic coupling 220 to transfer a large torque without slipping.However, in normal operation, drive shaft 210 and follower shaft 240typically spin rapidly to drive load 250, but only a low amount oftorque is required to keep follower shaft 240 spinning at the desiredhigh speed. As a result, the power supplied to the electromagnetic coilsmay be reduced to approximately 40-50% of P_(mx), saving electric power.In addition, in some embodiments, reducing the power to theelectromagnetic coils allows magnetic coupling 220 to disengage when thetorque applied to magnetic coupling 220 is too high, protecting valuablecomponents on the far side of magnetic coupling 220. For example, someembodiments of load 250 have a maximum rotational speed at which theycan safely operate. For example, in embodiments in which load 250 is anelectric generator, the generator may be damaged if operated at speedshigher than approximately 4000 revolutions per minute (RPM). If driveshaft 210 approaches an unsafe rotational speed, the amount of torquerequired to accelerate follower shaft 240 increases, exceeding theamount of torque that can be transmitted by magnetic coupling 220 whensupplied approximately 40-50% of P_(mx). As a result, magnetic coupling220 may transmit rotational motion when driven at a safe operationalspeed, but disengage if driven at a higher, unsafe speed.

FIG. 3 is a perspective view of an exemplary axial magnetic coupling220. As described above in connection with FIG. 2, axial magneticcoupling 220 includes drive array 222 and follower array 224, which areof approximately the same diameter, separated by barrier 230. Drivearray 222 includes an annular array of sections 310 a through 310 h,each of which includes a permanent magnet or electromagnetic coil thatproduces a magnetic flux in a particular direction. Similarly, followerarray 224 includes an annular array of sections 320 a through 320 h,each of which includes a permanent magnet or electromagnetic coil thatproduces a magnetic flux in a particular direction. Together, drivearray 222 and follower array 224 produce a magnetic field orientedsubstantially parallel to their common axis of rotation 226. Themagnetic field penetrates barrier 230 and couples drive array 222 tofollower array 224.

Arrows 330 indicate the orientation of the magnetic flux produced byeach particular section 310 a through 310 h or 320 a through 320 h. Asillustrated, sections 310 a through 310 h are arranged in a Halbacharray, in which the sections alternate between sections in which themagnetic flux is oriented normal to the downhole surface of magneticcoupling 220, such as sections 310 b and 310 d, and sections in whichthe magnetic flux is oriented transverse to such downhole surface, suchas sections 310 a and 310 c. In addition, as illustrated, theorientation of the magnetic flux in successive sections rotates in aconsistent direction as one proceeds around the annular array. Forexample, section 310 b of drive array 222 produces a magnetic fluxoriented uphole and substantially parallel to axis 226 of magneticcoupling 220, while section 310 d produces a magnetic flux orienteddownhole and substantially parallel to axis 226 of magnetic coupling220. In the alternating sections, for example, section 310 a of drivearray 222 produces a magnetic flux oriented transverse to axis 226, in aclockwise direction when viewed from the uphole surface of drive array222, while section 310 c of drive array 222 produces a magnetic fluxoriented transverse to axis 226, in a counter-clockwise direction whenviewed from the uphole surface of drive array 222. This arrangement ofmagnetized sections with rotating orientations increases the strength ofthe magnetic field on one side of the array while decreasing oreliminating the magnetic field on the other side of the array. In theembodiment illustrated, the magnetic field of drive array 222 isenhanced on the downhole face of drive array 222, which faces followerarray 224.

Sections 320 a through 320 h of follower array 224 are also arranged ina Halbach array, but in follower array 224 the direction in which theorientation of the magnetic flux in successive sections rotates isopposite to that in drive array 222. For example, in section 320 b, themagnetic flux is oriented uphole and substantially parallel to axis 226of magnetic coupling 220, as it is in the corresponding section 310 b ofdrive array 222. By contrast, in section 320 c, the magnetic flux isoriented transverse to axis 226, in a clockwise direction when viewedfrom the uphole surface of follower array 224, which is in the oppositedirection from that of corresponding section 310 c of drive array 222.As a result, the magnetic field of follower array 224 is enhanced on theuphole face of follower array 224, which faces drive array 222.

As a result of the enhancement of the magnetic fields on the facingsurfaces of drive array 222 and follower array 224, the maximum amountof torque transmitted through magnetic coupling 220 is increased. Forexample, in embodiments in which the magnetic field is substantiallyeliminated on the non-facing sides of each array, the maximum amount oftorque transmitted through magnetic coupling 220 may be approximatelydoubled.

In some embodiments, sections 310 a through 310 h of drive array 222 andsections 320 a through 320 h of follower array 224 include permanentmagnets. Because the temperature within wellbore 114 can be high, thepermanent magnets may include a material with a high magneticcoercivity, but whose magnetic flux density changes very little or notat all with increases in temperature. In particular, the permanentmagnets may have a high temperature coefficient of residual flux (Br)and intrinsic coercivity (Hcl) such as a Br and/or Hcl greater than−0.05%/C and −0.25%/C respectively. These materials exhibit littlechange to temperature, which makes them suitable for downholeapplications. For example, the permanent magnets in drive array 222 orfollower array 224 may include samarium cobalt.

In some embodiments drive array 222 or follower array 224 includeelectromagnetic coils, which produce the desired magnetic flux whenenergized. For example, in the embodiment illustrated in FIG. 3,follower array 224 includes electromagnetic coils in sections 320 athrough 320 h. Such electromagnetic coils may include cores that includesteel or other ferrous materials to increase the magnetic flux producedby the coils.

In some embodiments, follower array 224 may include a slip ring (notshown) that includes conductive material located on a surface offollower array 224. For example, a slip ring may be located on downholesurface 350 of follower array 224. The slip ring may be in electricalcontact with a brush (not shown) that provides electrical current toenergize electromagnetic coils in sections 320 a through 320 h offollower array 224. A second slip ring and brush may be used to providea return path for the electrical current. In such embodiments, the sliprings and brushes cannot be immersed in a conductive fluid, such asdrilling fluid, because the conductive fluid would create a shortcircuit between the brushes and prevent current from reaching andenergizing the electromagnetic coils. As a result, such embodimentsinclude barrier 230 to prevent drilling fluid from coming into contactwith the slip rings and brushes.

Although FIG. 3 illustrates an axial magnetic coupling that include aparticular number of sections 310 a through 310 h of drive array 222 andsections 320 a through 320 h of follower array 224, any suitable numberof sections may be used. For example, in some embodiments, drive array222 and follower array 224 may each include sixteen sections.Furthermore, although barrier 230 is illustrated in FIGS. 2 and 3 asenclosing follower array 224, follower shaft 240, and load 250, barrier230 may be arranged in any suitable fashion. For example, in someembodiments, barrier 230 encloses drive array 222 and drive shaft 210.

Although FIGS. 2 and 3 illustrate magnetic coupling 220 as an axialmagnetic coupling, in which drive array 222 and follower array 224 areof substantially similar diameter and magnetic field 228 between drivearray 222 and follower array 224 is substantially parallel to thearrays' axis of rotation 226, any suitable arrangement of arrays 222 and224 and field 228 may be used. For example, in some embodiments,magnetic coupling 220 is a radial magnetic coupling, in which followerarray 224 is of substantially smaller diameter and is placed within theinner diameter of drive array 222. Alternatively, drive array 222 may beof substantially smaller diameter and is placed within the innerdiameter of follower array 224. FIGS. 4-5, discussed in more detailbelow, illustrate an exemplary radial magnetic coupling.

FIG. 4 is an elevation section view of an exemplary radial magneticcoupling 400 that may be used in place of axial magnetic coupling 220.Radial magnetic coupling 400 includes drive array 422 and follower array424 separated by barrier 230. Unlike in the arrays in axial magneticcoupling 220, which have a substantially similar diameter, the arrays inradial magnetic coupling 400 are of unequal size, with one locatedwithin the other. For example, drive array 422 in radial magneticcoupling 400 may have an inner diameter larger than the outer diameterof follower array 424. Furthermore, follower array 424 may be locatedwithin the inner diameter of drive array 422, with drive array 422 andfollower array 424 sharing a common axis of rotation 226. Although FIG.4 illustrates drive array 422 having the larger diameter and followerarray 424 as located within the inner diameter of drive array 422, anyother suitable radial arrangement of the arrays may be used. Forexample, follower array 424 may have an inner diameter larger than theouter diameter of drive array 422, and drive array 422 may be locatedwithin the inner diameter of follower array 424.

As with axial magnetic coupling 220, drive array 422 includes an annulararray of sections, each of which includes a permanent magnet orelectromagnetic coil that produces a magnetic flux in a particulardirection to form a Halbach array. Similarly, follower array 424includes an annular array of sections, each of which includes apermanent magnet or electromagnetic coil that produces a magnetic fluxin a particular direction to form a Halbach array. The arrangement ofmagnets or electromagnetic coils in drive array 422 and follower array424 described in more detail in connection with FIG. 5 below

In some embodiments, drive array 422 and follower array 224 includepermanent magnets. Because the temperature within wellbore 114 can behigh, the permanent magnets may include materials with a high magneticcoercivity, but whose magnetic flux density changes very little or notat all with increases in temperature. In particular, the permanentmagnets may have a high temperature coefficient of residual flux (Br)and intrinsic coercivity (Hcl) such as a Br and/or Hcl greater than−0.05%/C and −0.25%/C respectively. These materials exhibit littlechange to temperature, which makes them suitable for downholeapplications. For example, the permanent magnets in drive array 422 orfollower array 424 may include samarium cobalt.

In some embodiments drive array 422 or follower array 424 includeelectromagnetic coils, which produce the desired magnetic flux whenenergized. For example, in the embodiment illustrated in FIG. 4,follower array 424 may include electromagnetic coils in sections 420 athrough 420 h. Such electromagnetic coils may include cores that includesteel or other ferrous materials to increase the magnetic flux producedby the coils.

In some embodiments, follower array 424 includes a slip ring (not shown)that includes conductive material located on a surface of follower array424. For example, a slip ring may be located on downhole surface 450 offollower array 424. The slip ring may be in electrical contact with abrush (not shown) that provides electrical current to energizeelectromagnetic coils in sections 420 a through 420 h of follower array424. A second slip ring and brush may be used to provide a return pathfor the electrical current. In such embodiments, the slip rings andbrushes cannot be immersed in a conductive fluid, such as drillingfluid, because the conductive fluid would create a short circuit betweenthe brushes and prevent current from reaching and energizing theelectromagnetic coils. As a result, such embodiments include barrier 230to prevent drilling fluid from coming into contact with the slip ringsand brushes.

Although barrier 230 is illustrated in FIG. 4 as enclosing followerarray 424 and follower shaft 240, barrier 230 may be arranged in anysuitable fashion. For example, in some embodiments, barrier 230 enclosesdrive array 422 and drive shaft 210.

FIG. 5 is a plan section view of exemplary radial magnetic coupling 400,cut along line A in FIG. 4.

Drive array 422 includes an annular array of sections 510 a through 510h, each of which includes a permanent magnet or electromagnetic coilthat produces a magnetic flux in a particular direction. Similarly,follower array 424 includes an annular array of sections 520 a through520 h, each of which includes a permanent magnet or electromagnetic coilthat produces a magnetic flux in a particular direction. Together, drivearray 422 and follower array 424 produce a magnetic field orientedsubstantially perpendicular to their common axis of rotation 226. Themagnetic field penetrates barrier 230 and couples drive array 422 tofollower array 424.

Arrows 530 indicate the orientation of the magnetic flux produced byeach particular section 510 a through 510 h or 520 a through 520 h. Asillustrated, sections 510 a through 510 h are arranged in a Halbacharray, in which the sections alternate between sections in which themagnetic flux is oriented normal to the inner surface of drive array422, such as sections 510 b and 510 d, and sections in which themagnetic flux is oriented transverse to such inner surface, such assections 510 a and 510 c. In addition, as illustrated, the orientationof the magnetic flux in successive sections rotates in a consistentdirection as one proceeds around the annular array. For example,sections 510 b and 510 f of drive array 422 each produce a magnetic fluxoriented inward toward axis 226, while sections 510 d and 510 h eachproduce a magnetic flux oriented outward away from axis 226. In thealternating sections, for example, sections 510 a and 510 e of drivearray 422 each produce a magnetic flux oriented transverse to axis 226,in a counter-clockwise direction when viewed from the uphole surface ofdrive array 422, while sections 510 c and 510 g of drive array 422 eachproduce a magnetic flux oriented transverse to axis 226, in a clockwisedirection when viewed from the uphole surface of drive array 422. Thisarrangement of magnetized sections with rotating orientations increasesthe strength of the magnetic field on one side of the array whiledecreasing or eliminating the magnetic field on the other side of thearray. In the embodiment illustrated, the magnetic field of drive array422 is enhanced on the inner surface of drive array 422, which faces theouter surface of follower array 424.

Sections 520 a through 520 h of follower array 224 are also arranged ina Halbach array, but here the direction in which the orientation of themagnetic flux in successive sections rotates is opposite to that indrive array 222. For example, in sections 520 b and 520 f, the magneticflux is oriented inward toward axis 226, as it is in the correspondingsections 510 b and 520 f of drive array 422. By contrast, in section 520c, the magnetic flux is oriented transverse to axis 226, in acounter-clockwise direction when viewed from the uphole surface offollower array 424, which is in the opposite direction from that ofcorresponding section 510 c of drive array 422. As a result, themagnetic field of follower array 224 is enhanced on the outer surface offollower array 424, which faces the inner surface of drive array 422.

In normal operation, the electrical current required to energizeelectromagnetic coils in a magnetic coupling may be provided by loadwhich is driven by the coupling. For example, load 250, discussed inconnection with FIG. 2, may include an electrical generator. Thegenerator may provide current to energize electromagnetic coils infollower array 224, discussed in connection with FIGS. 2 and 3. However,before magnetic coupling 220, discussed in connection with FIGS. 2 and3, begins to turn, the generator may produce no electrical current. As aresult, a separate power source is used to initially energize theelectromagnetic coils until a normal operating speed is reached.

FIG. 6 is a circuit diagram of an exemplary bootstrap circuit forenergizing electromagnetic coils in a magnetic coupling. For example,bootstrap circuit 600 may be used to energize electromagnetic coils infollower array 224, discussed in connection with FIGS. 2 and 3, or infollower array 424, discussed in connection with FIGS. 4 and 5. In theembodiment illustrated in FIG. 6, electromagnetic coils are present infollower array 424 of magnetic coupling 400. Circuit 600 allows abattery to initially energize the electromagnetic coils in followerarray 424 before magnetic coupling 400 begins to turn, then allow aseparate power source, such as a generator or alternator powered by themotion of follower array 424, to sustain the electromagnetic coilsduring normal operation.

Circuit 600 includes battery 610, which is electrically coupled todirect current to direct current (DC/DC) converter 620 through switch612. The positive terminal of DC/DC converter 620 is electricallycoupled to magnetic coupling 400 through node 622, diode 630, and node626. The negative terminal of DC/DC converter 620 is electricallycoupled to magnetic coupling 400 through node 624.

Circuit 600 also includes alternating current (AC) source 640. In someembodiments, AC source 640 is coupled through follower shaft 240 (notshown) to follower array 424, discussed in connection with FIGS. 4 and5. For example, load 250, discussed in connection with FIG. 2, mayinclude AC source 640. Current source 640 transforms the rotationalmotion of follower shaft 240 into electrical current. In someembodiments, AC source 640 is a generator, as discussed in connectionwith load 250 in FIG. 2. AC source 640 is electrically coupled to AC/DCconverter 650, which in turn is electrically coupled to DC/DC converter660. The positive terminal of DC/DC converter 620 is electricallycoupled to magnetic coupling 400 through node 662, diode 670, and node626. The negative terminal of DC/DC converter 660 is electricallycoupled to magnetic coupling 400 through node 664.

In operation, battery 610 supplies initial power to the electromagneticcoils in follower array 424 before magnetic coupling 400 begins to turn.When drive array 422 first begins to turn, switch 612 is closed,permitting current to flow from battery 610 to power electronics such asdirect current to direct current (DC/DC) converter 620. DC/DC converter620 produces a voltage V_(batt) at node 622 relative to node 624. As aresult, current flows from node 622 through diode 630 to node 626,through one or more electromagnetic coils in follower array 424,energizing the coils, through node 624 and back to DC/DC converter 620.As a result of the current flow through the coils, follower array 424magnetically couples to drive array 422 and thereby begins to turn inconjunction with drive array 422, turning follower shaft 240 (notshown).

When follower shaft 240 is turning, it supplies rotational motion toload 250, which may include AC source 640. AC source 640 transforms therotational motion of follower shaft 240 into electrical current. ACsource 640 supplies an alternating current to AC/DC converter 650. AC/DCconverter 650 in turn supplies a direct current to DC/DC converter 660.DC/DC converter 660 produces a voltage V_(gen) at node 662 relative tonode 664. When AC source 640 is spinning sufficiently rapidly, DC/DCconverter 660 may supply sufficient current that V_(gen) exceedsV_(batt). As a result, current flows from node 662 through diode 670 tonode 626, through one or more electromagnetic coils in follower array424, energizing the coils, through node 664 and back to DC/DC converter660. At this point, current from battery 610 is no longer used toenergize the coils, and switch 612 may be opened.

Although FIG. 6 illustrates circuit 600 including a radial magneticcoupling, any suitable magnetic coupling may be used. For example, anaxial magnetic coupling such as magnetic coupling 220, discussed inconnection with FIGS. 2 and 3, may be used in place of magnetic coupling400.

FIG. 7 is a flow chart of an exemplary method 700 for bootstrapping amagnetic coupling.

Method 700 may begin at step 710, in which current is supplied from abattery to an electromagnetic coil in a magnetic coupling. For example,as discussed above in connection with FIG. 6, when switch 612 is closed,battery 610 provides current to an electromagnetic coil in followerarray 424 of magnetic coupling 400 through DC/DC converter 620, diode630, and nodes 622, 624, and 626.

In step 720, the drive array and follower arrays in the magneticcoupling are coupled using the electromagnetic coil. As a result of thecurrent flow supplied in step 710, the electromagnetic coil in themagnetic coupling is energized and produces a magnetic field. Forexample, the electromagnetic coil in follower array 424 produces a fieldas described above in connection with FIGS. 5 and 6. This magnetic field(in conjunction with the field produced by magnetic sections in drivearray 422) couples drive array 422 and follower array 424.

In step 730, rotational motion is transferred to an AC source using themagnetic coupling. For example, rotational motion from turbine 202,discussed with reference to FIG. 2, may be transferred to load 250 usingdrive shaft 210, magnetic coupling 400, and follower shaft 240. Asdiscussed above in connection with FIG. 6, load 250 may include ACsource 640, which transforms that rotational motion into electricalcurrent.

In step 740, current is supplied from the AC source to theelectromagnetic coil. For example, as discussed above in connection withFIG. 6, AC source 640 provides current to the electromagnetic coil infollower array 424 through AC/DC converter 650, DC/DC converter 660,diode 670, and nodes 662, 664, and 666.

In step 750, the battery stops supplying current to the electromagneticcoil. For example, as discussed above in connection with FIG. 6, oncefollower shaft 240 is spinning sufficiently rapidly, AC source 640supplies sufficient current to energize the electromagnetic coils infollower array 424. As a result, battery 610 is disconnected by openingswitch 612.

Modifications, additions, or omissions may be made to method 700 withoutdeparting from the scope of the present disclosure. For example, theorder of the steps may be performed in a different manner than thatdescribed and some steps may be performed at the same time.Additionally, each individual step may include additional steps withoutdeparting from the scope of the present disclosure.

Embodiments disclosed herein include:

A. A magnetic coupling of a downhole tool that includes (a) a firstannular array of magnetic sections; (b) a second annular array ofmagnetic sections coupled to the first annular array by a magnetic fieldthat transfers rotational motion from the first annular array to thesecond annular array, and (c) a barrier disposed between the firstannular array and the second annular array, the barrier including anerosion-resistant layer.

B. A drilling system that includes (a) a drill string; and (b) amagnetic coupling located within the drill string, in which the magneticcoupling includes (c) a first annular array of magnetic sections, (d) asecond annular array of magnetic sections coupled to the first annulararray by a magnetic field that transfers rotational motion from thefirst annular array to the second annular array, and (e) a barrierdisposed between the first annular array and the second annular array,the barrier having an erosion-resistant layer; (f) a motor coupled tothe first annular array; and (g) a load coupled to the second annulararray.

C. A method of bootstrapping a magnetic coupling of a downhole tool thatincludes (a) rotating a first annular array of magnetic sections in amagnetic coupling; (b) supplying current from a battery to anelectromagnetic coil located within a second annular array of magneticsections in the magnetic coupling; (c) coupling the first annular arrayto the second annular array using a magnetic field produced by theelectromagnetic coil; (d) transferring rotational motion from the firstannular array to the second annular array using the magnetic field; (e)transferring rotational motion from the second annular array to analternating current (AC) source configured to transform rotationalmotion into electrical current; and (f) supplying electrical currentfrom the AC source to the electromagnetic coil.

Each of embodiments A, B, and C may have one or more of the followingadditional elements in any combination. Element 1: the first annulararray has a first outer diameter; the second annular array has a secondouter diameter approximately equal to the first outer diameter; thefirst annular array and the second annular array are configured torotate about a common axis of rotation; and the magnetic field isoriented approximately parallel to the common axis of rotation. Element2: the first annular array has an inner diameter; the second annulararray has an outer diameter smaller than the inner diameter; the secondannular array is disposed within the inner diameter of the first annulararray; the first annular array and the second annular array areconfigured to rotate about a common axis of rotation; and the magneticfield is oriented approximately perpendicular to the common axis ofrotation. Element 3: wherein the erosion-resistant layer includes alayer of titanium. Element 4: wherein the second annular array comprisesa plurality of permanent magnets. Element 5: wherein a magnet among theplurality of permanent magnets is a samarium cobalt magnet. Element 6:wherein the second annular array comprises a plurality ofelectromagnetic coils. Element 7: further comprising a bootstrap circuitfor energizing the plurality of electromagnetic coils, the bootstrapcircuit including (a) a battery; and (b) a first diode coupled to thebattery, the first diode permitting the battery to supply electricalcurrent to the plurality of electromagnetic coils. Element 8: thebootstrap circuit further including (c) a current source coupled to thesecond annular array, the current source configured to transformrotation of the second annular array into electrical current; and (d) asecond diode coupled to the current source, the second diode permittingthe current source to supply electrical current to the plurality ofelectromagnetic coils. Element 9: wherein the magnetic coupling includesa barrier disposed between the first annular array and the secondannular array, the barrier including an erosion-resistant layer.

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the disclosure as defined by the following claims.

What is claimed is:
 1. A magnetic coupling of a downhole toolcomprising: a first annular array of magnetic sections; a second annulararray of magnetic sections coupled to the first annular array by amagnetic field that transfers rotational motion from the first annulararray to the second annular array; and a barrier disposed between thefirst annular array and the second annular array, the barrier includingan erosion-resistant layer.
 2. The magnetic coupling of claim 1,wherein: the first annular array has a first outer diameter; the secondannular array has a second outer diameter approximately equal to thefirst outer diameter; the first annular array and the second annulararray are configured to rotate about a common axis of rotation; and themagnetic field is oriented approximately parallel to the common axis ofrotation.
 3. The magnetic coupling of claim 1, wherein: the firstannular array has an inner diameter; the second annular array has anouter diameter smaller than the inner diameter; the second annular arrayis disposed within the inner diameter of the first annular array; thefirst annular array and the second annular array are configured torotate about a common axis of rotation; and the magnetic field isoriented approximately perpendicular to the common axis of rotation. 4.The magnetic coupling of claim 1, wherein the erosion-resistant layerincludes a layer of titanium.
 5. The magnetic coupling of claim 1,wherein the second annular array comprises a plurality of permanentmagnets.
 6. The magnetic coupling of claim 5, wherein a magnet among theplurality of permanent magnets is a samarium cobalt magnet.
 7. Themagnetic coupling of claim 1, wherein the second annular array comprisesa plurality of electromagnetic coils.
 8. The magnetic coupling of claim7, further comprising a bootstrap circuit for energizing the pluralityof electromagnetic coils, the bootstrap circuit comprising: a battery;and a first diode coupled to the battery, the first diode permitting thebattery to supply electrical current to the plurality of electromagneticcoils.
 9. The magnetic coupling of claim 8, the bootstrap circuitfurther comprising: a current source coupled to the second annulararray, the current source configured to transform rotation of the secondannular array into electrical current; and a second diode coupled to thecurrent source, the second diode permitting the current source to supplyelectrical current to the plurality of electromagnetic coils.
 10. Adrilling system comprising: a drill string; a magnetic coupling locatedwithin the drill string, the magnetic coupling including: a firstannular array of magnetic sections; a second annular array of magneticsections coupled to the first annular array by a magnetic field thattransfers rotational motion from the first annular array to the secondannular array; a barrier disposed between the first annular array andthe second annular array, the barrier having an erosion-resistant layer;a motor coupled to the first annular array; and a load coupled to thesecond annular array.
 11. The drilling system of claim 10, wherein: thefirst annular array has a first outer diameter; the second annular arrayhas a second outer diameter approximately equal to the first outerdiameter; the first annular array and the second annular array areconfigured to rotate about a common axis of rotation; and the magneticfield is oriented approximately parallel to the common axis of rotation.12. The drilling system of claim 10 wherein: the first annular array hasan inner diameter; the second annular array has an outer diametersmaller than the inner diameter; the second annular array is disposedwithin the inner diameter of the first annular array; the first annulararray and the second annular array are configured to rotate about acommon axis of rotation; and the magnetic field is orientedapproximately perpendicular to the common axis of rotation.
 13. Thedrilling system of claim 10, wherein the erosion-resistant layerincludes a layer of titanium.
 14. The drilling system of claim 10,wherein the second annular array comprises a plurality of permanentmagnets.
 15. The drilling system of claim 10, wherein a magnet among theplurality of permanent magnets is a samarium cobalt magnet.
 16. Thedrilling system of claim 10, wherein the second annular array comprisesa plurality of electromagnetic coils.
 17. The drilling system of claim16, further comprising a bootstrap circuit for energizing the pluralityof electromagnetic coils, the bootstrap circuit comprising: a battery;and a first diode coupled to the battery, the first diode permitting thebattery to supply electrical current to the plurality of electromagneticcoils.
 18. The drilling system of claim 17, the bootstrap circuitfurther comprising: a current source coupled to the plurality ofelectromagnetic coils; and a second diode coupled to the current source,the second diode permitting the current source to supply current to theplurality of electromagnetic coils.
 19. The drilling system of claim 10,wherein the load is a generator.
 20. A method of bootstrapping amagnetic coupling of a downhole tool, comprising: rotating a firstannular array of magnetic sections in a magnetic coupling; supplyingcurrent from a battery to an electromagnetic coil located within asecond annular array of magnetic sections in the magnetic coupling;coupling the first annular array to the second annular array using amagnetic field produced by the electromagnetic coil; transferringrotational motion from the first annular array to the second annulararray using the magnetic field; transferring rotational motion from thesecond annular array to an alternating current (AC) source configured totransform rotational motion into electrical current; and supplyingelectrical current from the AC source to the electromagnetic coil. 21.The method of claim 20, wherein the magnetic coupling includes a barrierdisposed between the first annular array and the second annular array,the barrier including an erosion-resistant layer.
 22. The method ofclaim 20, wherein: the first annular array has a first outer diameter;the second annular array has a second outer diameter approximately equalto the first outer diameter; the first annular array and the secondannular array are configured to rotate about a common axis of rotation;and the magnetic field is oriented approximately parallel to the axis ofrotation.
 23. The method of claim 20, wherein: the first annular arrayhas an inner diameter; the second annular array has an outer diametersmaller than the inner diameter; the second annular array is disposedwithin the inner diameter of the first annular array; the first annulararray and the second annular array are configured to rotate about acommon axis of rotation; and the magnetic field is orientedapproximately perpendicular to the axis of rotation.